Light-reflective conductive particle, anisotropic conductive adhesive, and light-emitting device

ABSTRACT

A light-reflective conductive particle for an anisotropic conductive adhesive used for connecting a light-emitting element to a wiring board by anisotropic conductive connection includes a core particle covered with a metal material and a light reflecting layer formed of a light-reflective inorganic particle having a refractive index of 1.52 or greater on the surface of the core particle. Examples of the light-reflective inorganic particles having a refractive index of 1.52 or greater include a titanium oxide particle, a zinc oxide particle, and an aluminum oxide particle. The coverage of the light reflecting layer on the surface of the core particle is 70% or more.

TECHNICAL FIELD

The present invention relates to a light-reflective conductive particlefor an anisotropic conductive adhesive used for connecting alight-emitting element to a wiring board by anisotropic conductiveconnection, an anisotropic conductive adhesive containing the same, anda light-emitting device in which a light-emitting element is mounted ona wiring board using the adhesive.

BACKGROUND ART

A light-emitting device using a light-emitting diode (LED) element hasbeen widely used. An old-type light-emitting device has a structure, asshown in FIG. 4, in which an LED element 33 is bonded to a substrate 31with a die-bonding adhesive 32, a p electrode 34 and an n electrode 35on the upper surface of the LED element are wire-bonded to connectionterminals 36 on the substrate 31 with gold wires 37, and the entire LEDelement 33 is sealed with a transparent molding resin 38. However, inthe light-emitting device shown in FIG. 4, of light emitted from the LEDelement 33, the gold wires absorb light having a wavelength of 400 to500 nm which is emitted toward the upper surface side, and thedie-bonding adhesive 32 absorbs part of light which is emitted towardthe lower surface side. Therefore, there is a problem in that the lightemission efficiency of the LED element 33 is reduced.

In view of the above problems, as shown in FIG. 5, flip-chip mounting ofan LED element 33 has been proposed (Patent Literature 1). In thisflip-chip mounting technology, bumps 39 are formed on a p electrode 34and an n electrode 35, respectively, and a light reflecting layer 40 isprovided on a bump-formed surface of the LED element 33 so that the pelectrode 34 and the n electrode 35 are insulated from each other. TheLED element 33 and a substrate 31 are connected and fixed by curing ananisotropic conductive paste 41 or an anisotropic conductive film (notshown). Therefore, in the light-emitting device in FIG. 5, the lightemitted upward from the LED element 33 is not absorbed by the goldwires, and almost all light emitted downward is reflected by the lightreflecting layer 40 to be emitted upward. Therefore, light emissionefficiency (light extraction efficiency) is not reduced.

CITATION LIST Patent Literature

-   [Patent Literature 1] Japanese Patent Application Laid-Open No. Hei.    11-168235

SUMMARY OF THE INVENTION Problem(S) to be Solved by the Invention

However, in the technology of Patent Literature 1, the light reflectinglayer 40 needs to be provided on the LED element 33 by a metal vapordeposition method or the like so that the p electrode 34 and the nelectrode 35 are insulated, and therefore there is a problem in that anincrease in production cost is unavoidable.

On the other hand, when the light reflecting layer 40 is not provided,the surfaces of the conductive particles covered with gold, nickel, orcopper in the cured anisotropic conductive paste or anisotropicconductive film assume a brown or dark brown color. Further, an epoxyresin binder itself in which the conductive particles are dispersed alsoassumes a brown color due to an imidazole-based latent curing agentcommonly used to cure the binder. Therefore, it is difficult to improvethe light emission efficiency (light extraction efficiency) of lightemitted from the light-emitting element. In addition, there is a problemin which the light cannot be reflected while the color of light(emission color) is maintained as the original color.

It is an object of the present invention to solve the above problems ofthe conventional technology, and to provide a light-reflectiveconductive particle which does not reduce light emission efficiency anddoes not cause a hue difference between the emission color of alight-emitting element such as a light-emitting diode (LED) element andthe reflected light color even without providing a light reflectinglayer on the LED element during production of the light-emitting deviceby flip-chip mounting the light-emitting element on a wiring board usingan anisotropic conductive adhesive, the light reflecting layerincreasing the production cost. It is another object to provide ananisotropic conductive adhesive containing the light-reflectiveconductive particles, and a light-emitting device in which alight-emitting element is flip-chip mounted on a wiring board using theadhesive.

Means for Solving the Problem(S)

The present inventors have assumed that when a light-reflective functionis imparted to an anisotropic conductive adhesive itself, the lightemission efficiency could be prevented from being reduced. Under theassumption, a light reflecting layer of which color ranges from white togray and which is composed of specific inorganic particles is providedto the surface of conductive particles to be contained in theanisotropic conductive adhesive so that the coverage thereof is notlower than a predetermined coverage. As a result, the inventors havefound that the light emission efficiency of the light-emitting elementis not reduced and the hue difference is not caused between the emissioncolor of the light-emitting element and the reflected light color.Accordingly, the present invention has been completed.

The present invention provides a light-reflective conductive particlefor an anisotropic conductive adhesive used for connecting alight-emitting element to a wiring board by anisotropic conductiveconnection, wherein the light-reflective conductive particle includes acore particle covered with a metal material and a light reflecting layerformed of a light-reflective inorganic particle having a refractiveindex of 1.52 or greater on a surface of the core particle, wherein acoverage of the surface of the core particle covered with the lightreflecting layer is 700 or greater.

Further, the present invention provides an anisotropic conductiveadhesive used for connecting a light-emitting element to a wiring boardby anisotropic conductive connection, which is obtained by dispersingthe above-described light-reflective conductive particles in athermosetting resin composition which provides a cured product having alight transmittance (JIS K7105) of 80% or greater with respect tovisible light having a wavelength of 380 to 780 nm with a light pathlength of 1 cm.

Furthermore, the present invention provides a light-emitting device inwhich a light-emitting element is mounted on a wiring board with theanisotropic conductive adhesive interposed therebetween in a flip-chipmounting scheme.

Advantageous Effects of the Invention

The light-reflective conductive particle of the present invention for ananisotropic conductive adhesive used for connecting a light-emittingelement to a wiring board by anisotropic conductive connection includesa core particle covered with a metal material and a light reflectinglayer which is formed of light-reflective inorganic particles having arefractive index of 1.52 or greater on the surface of the core particleand of which color ranges from white to gray. In addition, the coverageof the surface of the core particle covered with the light reflectinglayer is 70% or greater. Further, the anisotropic conductive adhesive ofthe present invention is obtained by dispersing the light-reflectiveconductive particles in a thermosetting resin composition which providesa cured product having a light transmittance (JIS K7105) of 80% orgreater with respect to visible light having a wavelength of 380 to 780nm with a light path length of 1 cm. Therefore, in a light-emittingdevice obtained by flip-chip mounting a light-emitting element on awiring board using the anisotropic conductive adhesive of the presentinvention, even when the anisotropic conductive adhesive is cured,coloration does not occur. Further, since the contained light-reflectiveconductive particles have low wavelength dependency of reflectionproperty with respect to visible light, the light emission efficiency isincreased, and the emission color of reflected light from thelight-emitting element can be maintained as the original color.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional view of a light-reflective conductiveparticle of the present invention for an anisotropic conductiveadhesive.

FIG. 1B is a cross-sectional view of a light-reflective conductiveparticle of the present invention for an anisotropic conductiveadhesive.

FIG. 1C is a diagram showing a relationship between the lightreflectance and the mixed amount of titanium oxide.

FIG. 2 is a cross-sectional view of a light-emitting device of thepresent invention.

FIG. 3 is a diagram showing the light reflectance with respect towavelengths of cured products of anisotropic conductive adhesives ofExample 1 and Comparative Example 1.

FIG. 4 is a cross-sectional view of a conventional light-emittingdevice.

FIG. 5 is a cross-sectional view of a conventional light-emittingdevice.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

The present invention will be described in detail with reference to thedrawings.

FIGS. 1A and 1B are cross-sectional views of light-reflective conductiveparticles 10 and 20, respectively, of the present invention for ananisotropic conductive adhesive. The light-reflective conductiveparticle of FIG. 1A will be first described. The light-reflectiveconductive particle 10 includes a core particle 1 covered with a metalmaterial and a light reflecting layer 3 formed of light-reflectiveinorganic particles 2 having a refractive index of 1.52 or greater onthe Surface of the core particle.

The light-reflective inorganic particle 2 having a refractive index of1.52 or more is an inorganic particle taking on white under sunlight.Therefore, the color of the light reflecting layer 3 formed of theseparticles ranges from white to gray. This color ranging from white togray means that the wavelength dependency of reflection property withrespect to visible light is low and the visible light is likely to bereflected.

Examples of the preferable light-reflective inorganic particle 2 includeat least one type selected from a titanium oxide (TiO₂) particle, a zincoxide (ZnO) particle, and an aluminum oxide (Al₂O₃) particle. Whenphotodegradation of a cured product of a cured thermosetting resincomposition in the anisotropic conductive adhesive is concerned, among atitanium oxide particle, a zinc oxide particle, and an aluminum oxideparticle, a zinc oxide which has no catalytic property againstphotodegradation and also has a high refractive index can be preferablyused.

Since the core particle 1 is subjected to anisotropic conductiveconnection, the surface of the core particle is made of a metalmaterial. As an aspect of coating the surface with a metal material, anaspect in which the core particle 1 itself is a metal material, or anaspect in which the surface of a resin particle is covered with a metalmaterial is exemplified.

As the metal material, metal materials used for the conventionalconductive particle for anisotropic conductive connection can be used.Examples thereof include gold, nickel, copper, silver, solder,palladium, and aluminum particles, particles of alloys thereof, andparticles of a layered product thereof (for example, a layered nickelplating/gold flash plating product). Since gold, nickel, and copperamong them turn the conductive particle brown, they can provide theeffects of the present invention more effectively than other metalmaterials.

When the core particle 1 is prepared by covering a resin particle with ametal material, a resin particle portion of a resin particle coveredwith metal, which has been conventionally used as a conductive particlefor anisotropic conductive connection, can be used as the resinparticle. Examples of such a resin particle include styrene-based resinparticles, benzoguanamine resin particles, and nylon resin particles. Asa method for covering a resin particle with a metal material, anyconventionally known method can be used, and an electroless platingmethod, an electrolytic plating method, and the like can be used. Thethickness of the metal material layer for covering is enough to ensuregood connection reliability, and depends on a particle diameter of theresin particle or a kind of metal. It is generally 0.1 to 3 andpreferably 0.1 to 1 μm.

If the particle diameter of the core particle 1 having the surface ofthe metal material is too small, conduction failure tends to occur. Ifthe particle diameter is too large, a short circuit between traces tendsto occur. Therefore, the particle diameter is preferably 1 to 20 μm,more preferably 3 to 5 μm, and particularly preferably 3 to 5 μm. Inthis case, it is preferable that the shape of the core particle 1 bespherical. Further, the shape may be a flake-like shape or a rugby ballshape.

From the viewpoint of dimension relative to the particle diameter of thecore particle 1, if the thickness of the light reflecting layer 3 formedof the light-reflective inorganic particles 2 is too small relative tothe particle diameter of the core particle 1, the reflectance tends todecrease, and if the thickness is too large, conduction failure tends tooccur. Therefore, the thickness of the light reflecting layer ispreferably 0.5 to 50%, and more preferably 1 to 25%.

In the light-reflective conductive particle 10, if the particle diameterof the light-reflective inorganic particle 2 constituting the lightreflecting layer 3 is too small, light-reflective phenomenon tends notto occur, and if the particle diameter is too large, the formation of alight reflecting layer tends to be difficult. Therefore, the particlediameter is preferably 0.02 to 4 μm, more preferably 0.1 to 1 μm, andparticularly preferably 0.2 to 0.5 μl. In this case, from the viewpointof wavelength of light to be reflected, it is preferable that theparticle diameter of the light-reflective inorganic particle 2 be equalto or greater than 50% of the wavelength of light to be reflected (thatis, light emitted from the light-emitting element) so that the light tobe reflected does not permeate. Further, examples of the shape of thelight-reflective inorganic particle 2 include an amorphous shape, aspherical shape, a flaky shape, a needle shape, and the like. Amongthem, in terms of light diffusion effect, a spherical shape ispreferable. In terms of total reflection effect, the flaky shape ispreferable.

The light-reflective conductive particle 10 of FIG. 1A can be producedby a well-known film deposition technique (a so-called mechano-fusionmethod) in which powders of large size and small size are brought inphysically collision to form a film composed of particles with a smallparticle diameter on the surfaces of particles with a large particlediameter. In this case, the light-reflective inorganic particles 2 arefixed to the metal material on the surface of the core particle 1 so asto bite into the metal material. Further, since the inorganic particlesare less likely to be fixed to each other by fusion, a monolayer of theinorganic particles constitutes the light reflecting layer 3. Therefore,in the case of FIG. 1A, the thickness of the light reflecting layer 3may be equivalent to or slightly smaller than the particle diameter ofthe light-reflective inorganic particle 2.

The light-reflective conductive particle 20 of FIG. 1B will next bedescribed. The light-reflective conductive particle 20 is different fromthe light-reflective conductive particle 10 of FIG. 1A in that a lightreflecting layer 3 includes a thermoplastic resin 4 functioning as anadhesive, light-reflective inorganic particles 2 are fixed to each otherby the thermoplastic resin 4, and the light-reflective inorganicparticles 2 form multiple layers (for example, two or three layers).Since such a thermoplastic resin 4 is included, the mechanical strengthof the light reflecting layer 3 enhances, and therefore exfoliation ofthe inorganic particles is less likely to occur.

The light-reflective conductive particle 20 of FIG. 1B can be producedby the mechano-fusion method. In this case, a fine-particlethermoplastic resin 4 may be used in addition to the light-reflectiveinorganic particles 2 and the core particle 1. When the light-reflectiveconductive particle 20 of FIG. 1B is produced by the mechano-fusionmethod, the light-reflective conductive particle 10 of FIG. 1A issimultaneously produced.

As the thermoplastic resin 4, a halogen-free thermoplastic resin can bepreferably used for reduction of the load on the environment. Forexample, polyolefin such as polyethylene and polypropylene, polystyrene,acrylic resin, or the like can be preferably used.

Such a light-reflective conductive particle 20 can also be produced bythe mechano-fusion method. If the particle diameter of the thermoplasticresin 4 applicable for the mechano-fusion method is too small, an effectas an adhesive becomes low, and if the particle diameter is too large,the resin is difficult to be adhered to the core particle 1. Therefore,the particle diameter is preferably 0.02 to 4 μm, and more preferably0.1 to 1 μm. Further, if the amount added of the thermoplastic resin 4is too small, the effect as an adhesive is low, and if the amount is toolarge, unintended particle aggregates are produced. Therefore, theamount added is preferably 0.2 to 500 parts by mass, based on 100 partsby mass of the core particle 1 and more preferably 4 to 25 parts bymass.

In the light-reflective conductive particles 10 and 20, if the coverageof the surface of the core particle 1 covered with the light reflectinglayer 3 including the light-reflective inorganic particles 2 as acomponent is too low, there is a concern that a desired lightreflectivity cannot be obtained. The coverage is 70% or greater, andpreferably 80% or greater.

The coverage can be determined, for example, by arranginglight-reflective conductive particles on an adhesive tape so as to forma single particle layer, capturing an image taken with a CCD camera as aplan image enlarged at a magnification of 100 into a personal computer,and calculating an average coverage on the basis of a predeterminedbinarization process (process of converting a gray image into a binaryimage) by an image processing software (coverage measurement A). Thecoverage can also be determined by enlarging 30 conductive particleswhich are arbitrarily selected by a scanning electron microscope (forexample, at a magnification of 8000), drawing a state of each conductiveparticle covered with adhered insulating particles in a two-dimensionalmanner, and calculating the average coverage of the 30 particles(coverage measurement B). Furthermore, the coverage can also bedetermined by empirically determining that the coverage is equal to ormore than the lowest coverage (coverage measurement C).

It should be noted that the color of a surface of a core particle havingthe surface of gold, nickel, or copper is brown. In contrast, as aqualitative and simplified criterion for empirically determining thecoverage, a criterion in which a case where the core particle can beobserved to be gray is considered to be a coverage of 70% or greater canbe exemplified. This criterion is supported by the general finding inwhich the correlation between the reflectance of a coating film of apaint including a resin material and light-reflective inorganicparticles and the coverage is very high.

For example, titanium oxide powders having an average particle diameterof 0.5 μm is homogeneously mixed in a thermosetting epoxy bindercomposition used in Example 1 of the present description in an amountsuch that the ratio of the titanium oxide powder to the total of thethermosetting epoxy binder composition and the titanium oxide powder is0%, 40%, 50%, 60%, 70%, 80%, 90%, or 100%, as shown in FIG. 1C, toprepare a paint. The paint is applied to a black board so that the driedthickness is 100 μm, heated at 200° C. for one minute, and cured toobtain a cured product. The reflectance (JIS K7105) of light having awavelength of 450 nm with respect of the resultant cured product ismeasured with a spectrophotometer (U3300, Hitachi, Ltd.) to obtain areflectance curve shown in FIG. 1C. As observed from FIG. 1C, the ratioof the titanium oxide powder to the total of the thermosetting epoxybinder composition and the titanium oxide powder abruptly increases at70%.

The anisotropic conductive adhesive of the present invention will nextbe described. The anisotropic conductive adhesive is obtained bydispersing the light-reflective conductive particles of the presentinvention in a thermosetting resin composition, and can be a paste, afilm, or the like.

As a thermosetting resin composition used for the anisotropic conductiveadhesive of the present invention, it is preferable that the usedthermosetting resin composition be as colorless and transparent aspossible. This is because the light reflective efficiency of thelight-reflective conductive particles in the anisotropic conductiveadhesive does not decrease, and the incident light is reflected withoutchanging the color of the incident light. The colorless and transparentherein means that the cured product of the anisotropic conductiveadhesive has a light transmittance (JIS K7105) of 80% or more withrespect to visible light having a wavelength of 380 to 780 nm with alight path length of 1 cm, and preferably 90% or more.

In the anisotropic conductive adhesive of the present invention, if theamount added of the light-reflective conductive particles based on 100parts by mass of the thermosetting resin composition is too small,conductive failure tends to occur, and if the amount is too large, ashort circuit between traces tends to occur. Therefore, the amount addedis preferably 1 to 100 parts by mass, and more preferably 10 to 50 partsby mass.

In order to improve light emission efficiency of the light-emittingelement, the reflection property of the anisotropic conductive adhesiveof the present invention is desirably configured such that thereflectance (JIS K 7105) of the cured product of the anisotropicconductive adhesive having a thickness of 100 μm with respect to lighthaving a wavelength of 450 nm be at least 15%. In order to obtain such areflectance, the reflection property and the amount added of the usedlight-reflective conductive particle, the mixed composition of thethermosetting resin composition, and the like may be appropriatelyadjusted. In general, increase of the amount added of thelight-reflective conductive particles having a favorable reflectionproperty tends to increase the reflectance.

From the viewpoint of refractive index (JIS K7142), the reflectionproperty of the anisotropic conductive adhesive can be evaluated. Thisis because a too large difference of refractive index between thethermosetting resin composition of the anisotropic conductive adhesiveand the light-reflective conductive particle increases a lightrefractive amount on an interface between the light-reflectiveconductive particle and the cured product of the thermosetting resincomposition surrounding the particle. Specifically, it is desirable thatthe difference of refractive index between the thermosetting resincomposition and the light-reflective inorganic particle is 0.02 orgreater, and preferably 0.3 or greater. Further, the reflective index ofthe thermosetting resin composition containing an epoxy resin as a maincomponent is generally about 1.5.

As the thermosetting resin composition constituting the anisotropicconductive adhesive of the present invention, materials used in theconventional anisotropic conductive adhesive and anisotropic conductivefilm can be used. In general, such a thermosetting resin composition isobtained by mixing a curing agent to an insulating binder resin.Preferable examples of the insulating binder resin include epoxy resinscontaining an alicyclic epoxy resin, a heterocyclic epoxy compound, or ahydrogenated epoxy resin as a main component.

Preferable examples of the alicyclic epoxy compound include compoundshaving two or more epoxy groups in their molecules. These may be liquidor solid. Specific examples thereof include glycidyl hexahydrobisphenolA, 3,4-epoxycyclohexenylmethyl-3′,4′-epoxycyclohexene carboxylate, andthe like. Among these, it is preferable that glycidyl hexahydrobisphenolA and 3,4-epoxycyclohexenylmethyl-3′,4′-epoxycyclohexene carboxylate beused in terms that the cured product can ensure the light transmittancesuitable for mounting an LED element and rapid curing properties areexcellent.

Examples of the heterocyclic epoxy compound include epoxy compoundshaving a triazine ring. In particular,1,3,5-tris(2,3-epoxypropyl)-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione ispreferable.

As the hydrogenated epoxy compound, hydrogenated compounds of the abovealicyclic epoxy compound or heterocyclic epoxy compound, or other knownhydrogenated epoxy resins can be used.

In the present invention, the alicyclic epoxy compound, the heterocyclicepoxy compound, and the hydrogenated epoxy compound may be used alone,or two or more kinds thereof may be used in combination. Any other epoxycompound may be used in combination with these epoxy compounds as longas the effect of the present invention is impaired. Examples thereofinclude glycidyl ethers obtained by reacting epichlorohydrin with apolyhydric phenol such as bisphenol A, bisphenol F, bisphenol S,tetramethylbisphenol A, diarylbisphenol A, hydroquinone, catechol,resorcin, cresol, tetrabromobisphenol A, trihydroxybiphenyl,benzophenone, bisresorcinol, bisphenol hexafluoroacetone,tetramethylbisphenol A, tetramethylbisphenol F,tris(hydroxyphenyl)methane, bixylenol, phenol-novolac, and cresolnovolac; polyglycidyl ethers obtained by reacting epichlorohydrin withan aliphatic polyhydric alcohol such as glycerol, neopentyl glycol,ethylene glycol, propylene glycol, butylene glycol, hexylene glycol,polyethylene glycol, and polypropylene glycol; glycidyl ether estersobtained by reacting epichlorohydrin with a hydroxycarboxylic acid suchas p-oxybenzoic acid and β-oxynaphthoic acid; polyglycidyl estersobtained from polycarboxylic acids such as phthalic acid, methylphthalicacid, isophthalic acid, terephthalic acid, tetrahydrophthalic acid,endomethylene tetrahydrophthalic acid, endomethylene hexahydrophthalicacid, trimellitic acid, and polymerized fatty acids;glycidylaminoglycidyl ethers obtained from aminophenol andaminoalkylphenol; glycidylaminoglycidyl esters obtained fromaminobenzoic acid; glycidylamines obtained from aniline, toluidine,tribromoaniline, xylylenediamine, diamino cyclohexane,bisaminomethylcyclohexane, 4,4′-diaminodiphenyl methane,4,4′-diaminodiphenyl sulfone, and the like; and known epoxy resins suchas epoxidized polyolefin.

As the curing agent, an acid anhydride, an imidazole compound, dicyan,and the like can be used. Among them, an acid anhydride which isdifficult to change the color of the cured product, and particularly analicyclic acid anhydride curing agent can be preferably used.Specifically, methylhexahydrophthalic anhydride (MeHHPA), and the likecan preferably be used.

In the case where an alicyclic epoxy compound and an alicyclic acidanhydride curing agent are used in the thermosetting resin compositionof the anisotropic conductive adhesive of the present invention, if theused amount of the alicyclic acid anhydride curing agent is too small,the amount of an uncured epoxy compound tends to be large, resulting inpoor curing. If the used amount is too large, corrosion of a material tobe adhered tends to be promoted by the effects of excess curing agent.Therefore, it is preferable that the alicyclic acid anhydride curingagent be used in a ratio of 80 to 120 parts by mass based on 100 partsby mass of the alicyclic epoxy compound, and more preferably 95 to 105parts by mass.

The anisotropic conductive adhesive of the present invention can beproduced by homogeneously mixing the light-reflective conductiveparticles and the thermosetting resin composition. If the anisotropicconductive adhesive is used as an anisotropic conductive film, theparticles and the resin composition are dispersed and mixed in a solventsuch as toluene, and the resultant mixture is applied to a PET filmsubjected to a peel treatment so as to be a desired thickness, and driedat about 80° C.

The light-emitting device of the present invention will next bedescribed with reference to FIG. 2. A light-emitting device 200 is alight-emitting device in which the above-described anisotropicconductive adhesive of the present invention is applied betweenconnection terminals 22 on a substrate 21 and bumps 26 for connectionwhich are respectively formed on an n electrode 24 and a p electrode 25of an LED element 23 as a light-emitting element, and the substrate 21and the LED element 23 are subjected to flip-chip mounting. A curedproduct 100 of the anisotropic conductive adhesive is obtained bydispersing light-reflective conductive particles 10 in the cured product11 of the thermosetting resin composition. If necessary, the LED element23 may be sealed with a transparent molding resin so as to cover theentire element 23.

In the light-emitting device 200 having such a configuration, among thelight emitted by the LED element 23, light emitted toward the substrate21 side is reflected by the light-reflective conductive particles 10 inthe cured product 100 of the anisotropic conductive adhesive, and lightis emitted from the upper surface of the LED element 23. Therefore,reduction of the light emission efficiency can be prevented.

EXAMPLES Example 1 (Formation of Light-Reflective Conductive Particle)

4 parts by mass of titanium oxide powder (KR-380, Titan kogyo, Ltd.)having an average particle diameter of 0.5 μm and 20 parts by mass ofAu-covered resin conductive particles of which the appearance color wasbrown and the average particle diameter was 5 μm (particle in which aspherical acrylic resin particle having an average particle diameter of4.6 μm was subjected to electroless gold plating so that the goldplating has a thickness of 0.2 μm: BRIGHT 20GNB4.6EH, Nippon ChemicalIndustrial Co., Ltd.) were added to a mechano-fusion device (AMS-GMP,Hosokawa Micron Corporation). A light reflecting layer which was formedof the titanium oxide particles and had a thickness of about 0.3 μm wasformed on the surface of the conductive particles under conditions of arotational speed of 1000 rpm and a rotational time of 20 minutes, toobtain a light-reflective conductive particle of Example 1. Theappearance color of the light-reflective conductive particle was gray.

(Measurement of Coverage)

The coverage of the obtained light-reflective conductive particlecovered with the light reflecting layer was measured in accordance withthe coverage measurement B. The obtained results are shown in Table 1.

(Measurement of Light Reflectance)

15 parts by mass of the light-reflective conductive particles obtainedand 100 parts by mass of a colorless and transparent thermosetting epoxybinder composition (YX-8000, Mitsubishi Chemical Corp.) having arefractive index of about 1.5 were homogeneously mixed using a vacuumdefoaming mixer to obtain an anisotropic conductive adhesive of Example1.

The obtained anisotropic conductive adhesive was applied to a ceramicwhite plate so as to have a dried thickness of 100 μm, heated at 200° C.for one minute, and cured. The reflectance (JIS K 7105) of the curedproduct with respect to light having a wavelength of 450 nm was measuredusing a spectrophotometer (U3300, Hitachi, Ltd.). The obtained resultsare shown in Table 1 and FIG. 3.

(Electrical Properties (Conduction Reliability, Insulation Reliability)Evaluation Test)

A test IC chip (conductor connection area/conductor−space=1600 μm²/50μmP) having a square of side 6 mm and a gold bump of height 15 μm wasflip-chip mounted on a glass epoxy substrate having wirings, in whichcopper wirings having a pitch of 50 μm were Ni/Au plated (5.0 μm/0.3 μmin thickness), using the anisotropic conductive adhesive prepared duringthe light reflectance evaluation test under conditions of 200° C., 60seconds, and 1 Kg/chip, thereby obtaining a test IC module.

1. Conduction Reliability

The test IC module was subjected to a temperature cycle test (TCT) (JISC5030) of alternating between heating at a high temperature (100° C.)and cooling at a low temperature (−40° C.) to measure resistance valuesat an initial stage and after 500 cycles by the four probe method. Whenthe resistance value was smaller than 1Ω, the conduction reliability wasevaluated as good (A). When the resistance value was 1Ω or larger, theconduction reliability was evaluated as poor (C). The obtained resultsare shown in Table 1.

2. Insulation Reliability

A separately produced test IC module was subjected to the aging testunder an environment of 85° C. and 85% RH over 1000 hours to measure theresistance values at an initial stage and after 1000 hours. When theresistance value was 10⁶Ω or larger, the insulation reliability wasevaluated as good (A). When the resistance value was smaller than 10⁶Ω,the insulation reliability was evaluated as poor (C). The obtainedresults are shown in Table 1.

Example 2

A light-reflective conductive particle of which the appearance color wasgray was obtained in the same manner as in Example 1 except that therotational speed of the mechano-fusion device (AMS-GMP, Hosokawa MicronCorporation) was changed from 1000 rpm to 700 rpm and the rotation timewas changed from 20 minutes to 10 minutes, and further an anisotropicconductive adhesive was obtained. In the same manner as in Example 1,the coverage and the reflectance were measured, and the electricalproperties (conduction reliability and insulation reliability)evaluation test was performed. The obtained results are shown in Table1.

Example 3

A light-reflective conductive particle of which the appearance color wasgray was obtained in the same manner as in Example 1 except thatNi-covered resin conductive particles (52NR-4.6EH, Nippon ChemicalIndustrial Co., Ltd.) having an average particle diameter of 5.0 μm wereused instead of the Au-covered resin conductive particles, and furtheran anisotropic conductive adhesive was obtained. In the same manner asin Example 1, the coverage and the reflectance were measured, and theelectrical properties (conduction reliability and insulationreliability) evaluation test was performed. The obtained results areshown in Table 1.

Example 4 4 parts by mass of titanium oxide powder (KR-380, Titan kogyo,Ltd.) having an average particle diameter of 0.5 μm, 3 parts by mass ofpolystyrene (PS) particles (GROSSDELL 204S, Mitsui Chemicals, Inc.)having an average particle diameter of 0.2 μm as an adhesive particle,and 20 parts by mass of conductive particles having an average particlediameter of 5 μm (particle in which a spherical acrylic resin particlehaving an average particle diameter of 4.6 μm was subjected toelectroless gold plating so that the gold plating has a thickness of 0.2μm: BRIGHT 20GNB4.6EH, Nippon Chemical Industrial Co., Ltd.) were addedto a mechano-fusion device (AMS-GMP, Hosokawa Micron Corporation). Alight reflecting layer which was formed of the styrene and titaniumoxide particles and had a thickness of about 1 μm was formed on thesurface of the conductive particles under conditions of a rotationalspeed of 1000 rpm and a rotational time of 20 minutes, to obtain alight-reflective conductive particle of which the appearance color wasgray. Further, an anisotropic conductive adhesive was obtained. In thesame manner as in Example 1, the coverage and the reflectance weremeasured, and the electrical properties (conduction reliability andinsulation reliability) evaluation test was performed. The obtainedresults are shown in Table 1. Example 5

A light-reflective conductive particle of which the appearance color wasgray was obtained in the same manner as in Example 4 except thatNi-covered resin conductive particles (52NR-4.6EH, Nippon ChemicalIndustrial Co., Ltd.) having an average particle diameter of 5.0 μm wereused instead of the Au-covered resin conductive particles. Further, ananisotropic conductive adhesive was obtained. In the same manner as inExample 1, the coverage and the reflectance were measured, and theelectrical properties (conduction reliability and insulationreliability) evaluation test was performed. The obtained results areshown in Table 1.

Example 6

A light-reflective conductive particle of which the appearance color wasgray was obtained in the same manner as in Example 4 except thatpolyethylene (PE) particles (AMIPEARL WF300, Mitsui Chemicals, Inc.)having an average particle diameter of 0.2 μm was used instead of thepolystyrene particles (GROSSDELL 204S, Mitsui Chemicals, Inc.) having anaverage particle diameter is 0.2 μm. Further, an anisotropic conductiveadhesive was obtained. In the same manner as in Example 1, the coverageand the reflectance were measured, and the electrical properties(conduction reliability and insulation reliability) evaluation test wasperformed. The obtained results are shown in Table 1.

Example 7

A light-reflective conductive particle of which the appearance color wasgray was obtained in the same manner as in Example 4 except that a zincoxide powder (one type of zinc oxide, Hakusuitech Ltd.) having anaverage particle diameter of 0.5 μm was used instead of the titaniumoxide powder having an average particle diameter of 0.5 μl. Further, ananisotropic conductive adhesive was obtained. In the same manner as inExample 1, the coverage and the reflectance were measured, and theelectrical properties (conduction reliability and insulationreliability) evaluation test was performed. The obtained results areshown in Table 1.

Example 8

A light-reflective conductive particle of which the appearance color wasgray was obtained in the same manner as in Example 4 except that analuminum oxide powder (AE-2500SI, Admatechs Company Limited) having anaverage particle diameter of 0.5 μm was used instead of the titaniumoxide powder having an average particle diameter of 0.5 μm. Further, ananisotropic conductive adhesive was obtained. In the same manner as inExample 1, the coverage and the reflectance were measured, and theelectrical properties (conduction reliability and insulationreliability) evaluation test was performed. The obtained results areshown in Table 1.

Example 9

A light-reflective conductive particle of which the appearance color wasgray was obtained in the same manner as in Example 4 except thatmagnesium carbonate having an average particle diameter of 0.5 μm wasused instead of the titanium oxide powder having an average particlediameter of 0.5 μm. Further, an anisotropic conductive adhesive wasobtained. In the same manner as in Example 1, the coverage and thereflectance were measured, and the electrical properties (conductionreliability and insulation reliability) evaluation test was performed.The obtained results are shown in Table 1.

Example 10

A light-reflective conductive particle of which the appearance color wasgray was obtained in the same manner as in Example 4 except that atitanium oxide powder (JR405, Tayca Corporation) having an averageparticle diameter of 0.2 μm was used instead of the titanium oxidepowder having an average particle diameter of 0.5 μm. Further, ananisotropic conductive adhesive was obtained. In the same manner as inExample 1, the coverage and the reflectance were measured, and theelectrical properties (conduction reliability and insulationreliability) evaluation test was performed. The obtained results areshown in Table 1.

Comparative Example 1

Au-covered resin conductive particles of which the appearance color wasbrown (particle in which a spherical acrylic resin particle having anaverage particle diameter of 4.6 μm was subjected to electroless goldplating so that the gold plating has a thickness of 0.2 μm: BRIGHT20GNB4.6EH, Nippon Chemical Industrial Co., Ltd.) were used and the sameprocedure as in Example 1 was performed to obtain an anisotropicconductive adhesive. In the same manner as in Example 1, the reflectancewas measured, and the electrical properties (conduction reliability andinsulation reliability) evaluation test was performed. The obtainedresults are shown in Table 1.

Comparative Example 2

An anisotropic conductive adhesive was obtained in the same manner as inComparative Example 1 except that Ni-covered resin conductive particles(52NR-4.6EH, Nippon Chemical Industrial Co., Ltd.) of which theappearance color was brown and the average particle diameter was 5.0 μmwere used instead of the Au-covered resin conductive particles. In thesame manner as in Example 1, the reflectance was measured, and theelectrical properties (conduction reliability and insulationreliability) evaluation test was performed. The obtained results areshown in Table 1.

Comparative Example 3

3 parts by mass of polystyrene-based particles (GROSSDELL 204S, MitsuiChemicals, Inc.) having an average particle diameter of 0.2 μm and 20parts by mass of conductive particles having an average particlediameter of 5 μm (particle in which a spherical acrylic resin particlehaving an average particle diameter of 4.6 μm was subjected toelectroless gold plating so that the gold plating has a thickness of 0.2μm: BRIGHT 20GNB4.6EH, Nippon Chemical Industrial Co., Ltd.) were addedto the mechano-fusion device (AMS-GMP, Hosokawa Micron Corporation). Astyrene layer having a thickness of 0.2 μm was formed on the surface ofthe conductive particles under conditions of a rotational speed of 1000rpm and a rotational time of 20 minutes. Resin-covered conductiveparticles of which the appearance color was brown were thereby obtained.Further, an anisotropic conductive adhesive was obtained. In the samemanner as in Example 1, the reflectance was measured, and the electricalproperties (conduction reliability and insulation reliability)evaluation test was performed. The obtained results are shown in Table1.

Comparative Example 4

A conductive particle of which the appearance color was brown wasobtained in the same manner as in Example 4 except that silicon oxide(silica) powder (SEAHOSTAR KEP-30, NIPPON SHOKUBAI CO., LTD.) having arefractive index of 1.45 and an average particle diameter of 0.5 μm wasused instead of the titanium oxide powder having an average particlediameter of 0.5 μm. Further, an anisotropic conductive adhesive wasobtained. In the same manner as in Example 1, the coverage and thereflectance were measured, and the electrical properties (conductionreliability and insulation reliability) evaluation test was performed.The obtained results are shown in Table 1.

TABLE 1 Example Comparative Example 1 2 3 4 5 6 7 8 9 10 1 2 3 4 Rawmaterial Surface metal Au Au Ni Au Ni Au Au Au Au Au Au Ni Au Auconductive Particle diameter (μm) 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.05.0 5.0 5.0 5.0 5.0 particle Adhesive Type — — — PS PS PE PS PS PS PS —— PS PS particle Particle diameter (μm) — — — 0.2 0.2 0.1 0.2 0.2 0.20.2 — — 0.2 0.2 Inorganic Type TiO₂ TiO₂ TiO₂ TiO₂ TiO₂ TiO₂ ZnO Al₂O₃MgCO₃ TiO₂ — — — SiO₂ particle Particle diameter (μm) 0.5 0.5 0.5 0.50.5 0.5 0.5 0.5 0.5 0.2 — — — 0.5 Refractive index n1 2.71 2.71 2.712.71 2.71 2.71 1.95 1.76 1.52 2.71 — — — 1.45 Light- Color Gray GrayGray Gray Gray Gray Gray Gray Gray Gray Brown Black Brown Brownreflective Coverage (%) >90 75 >90 >90 >90 >90 >90 >90 >90 >90 — — — >90conductive Reflectance (%) 30 20 32 30 32 30 22 17 15 30 8 9 8 9particle Conduction Initial stage A A A A A A A A A A A A A Areliability After TCT A A A A A A A A A A A A A A Insulation Initialstage A A A A A A A A A A C C A A reliability After aging A A A A A A AA A A C C C A

As seen from Table 1, in the anisotropic conductive adhesives using thelight-reflective conductive particles of Examples 1 to 10, the coveragesof the light reflecting layer are 70% or greater, the light reflectancesare all 15% or greater, and blue light having a wavelength of 450 nm wasreflected with the original color being maintained. The conductionreliabilities and the insulation reliabilities thereof are alsofavorable.

On the other hand, in Comparative Examples 1 to 3, since the lightreflecting layer is not provided on the surfaces of the conductiveparticles, the light reflectance is smaller than 10%. In ComparativeExamples 1 and 2, short circuit was caused at the initial stage. InComparative Example 3, short circuit was caused after the aging.Therefore, there is a problem of insulation reliability. In ComparativeExample 4, the coverage of the light reflecting layer is 70% or greater.However, since silicon oxide having a refractive index smaller than 1.52is used as an inorganic particle, the color of the conductive particlesafter formation of an inorganic particle layer is brown, and the lightreflectance is about 10%. This may be because the difference ofrefractive index between the silicon oxide and the binder composition ofthe anisotropic conductive adhesive is smaller than 0.02.

INDUSTRIAL APPLICABILITY

In the light-reflective conductive particle of the present invention,while a light reflecting layer which increases the production costduring the production of a light-emitting device by flip-chip mountinglight-emitting elements such as a light-emitting diode (LED) element ona wiring board using an anisotropic conductive adhesive is not providedon the light-emitting element, the light emission efficiency is notreduced, and a hue difference between the emission color of thelight-emitting element and the reflected light color cannot be caused.Therefore, the anisotropic conductive adhesive of the present inventionis useful for flip-chip mounting of LED elements.

REFERENCE SIGNS LIST

-   1 core particle-   2 light-reflective inorganic particle-   3 light reflecting layer-   4 thermoplastic resin-   10, 20 light-reflective conductive particle-   11 cured product of thermosetting resin composition-   21 substrate-   22 connection terminal-   23 LED element-   24 n electrode-   25 p electrode-   26 bump-   100 cured product of anisotropic conductive adhesive-   200 light-emitting device

1. A light-reflective conductive particle for an anisotropic conductiveadhesive used for connecting a light-emitting element to a wiring boardby anisotropic conductive connection, wherein the light-reflectiveconductive particle comprises a core particle covered with a metalmaterial and a light reflecting layer formed of a light-reflectiveinorganic particle having a refractive index of 1.52 or greater on asurface of the core particle, and a coverage of the surface of the coreparticle covered with the light reflecting layer is 70% or greater. 2.The light-reflective conductive particle according to claim 1, whereinthe metal material with which the core particle is covered is gold,nickel or copper.
 3. The light-reflective conductive particle accordingto claim 1, wherein the core particle itself is a gold, nickel or copperparticle.
 4. The light-reflective conductive particle according to claim1, wherein the core particle is a particle fowled from a resin particlecovered with gold, nickel or copper.
 5. The light-reflective conductiveparticle according to claim 1, wherein the core particle has a particlediameter of 1 to 20 μm, and the light reflecting layer has a thicknessof 0.5 to 50% of the particle diameter of the core particle.
 6. Thelight-reflective conductive particle according to claim 1, wherein thelight-reflective inorganic particle is at least one type selected from atitanium oxide particle, a zinc oxide particle, and an aluminum oxideparticle.
 7. The light-reflective conductive particle according to claim1, wherein the light reflecting layer includes a thermoplastic resin. 8.The light-reflective conductive particle according to claim 7, whereinthe thermoplastic resin is a polyolefin.
 9. An anisotropic conductiveadhesive used for connecting a light-emitting element to a wiring boardby anisotropic conductive connection, the anisotropic conductiveadhesive being obtained by dispersing the light-reflective conductiveparticle according to claim 1 in a thermosetting resin composition whichprovides a cured product having a light transmittance (JIS K7105) of 80%or greater with respect to visible light having a wavelength of 380 to750 nm with a light path length of 1 cm.
 10. The anisotropic conductiveadhesive according to claim 9, wherein the light-reflective conductiveparticle is added in an amount of 1 to 100 parts by mass based on 100parts by mass of the thermosetting resin composition.
 11. Theanisotropic conductive adhesive according to claim 9, wherein the curedproduct of the anisotropic conductive adhesive having a thickness of 100μm has a reflectance (JIS K7105) of at least 15% with respect to lighthaving a wavelength of 450 nm.
 12. The anisotropic conductive adhesiveaccording to claim 9, wherein a difference of refractive index betweenthe thermosetting resin composition and the light-reflective conductiveparticle is 0.02 or greater.
 13. The anisotropic conductive adhesiveaccording to claim 9, wherein the thermosetting resin compositionincludes an epoxy resin and an acid anhydride-based curing agent.
 14. Alight-emitting device in which a light-emitting element is mounted on awiring board with the anisotropic conductive adhesive according to claim9 interposed therebetween in a flip-chip mounting scheme.
 15. Thelight-emitting device according to claim 14, wherein the light-emittingelement is a light-emitting diode.